1. Field of the Invention
This invention relates to a pressure-sensitive adhesive article, such as a tape, comprising a backing and a pressure-sensitive adhesive coating. The adhesive coating comprises microspheres chemically bonded to a cross-linked polymer network.
2. Brief Description of the Related Art
Transparent imaging sheets (or films) are widely used as overhead transparencies for overhead projectors during visual presentations. These transparent imaging sheets are processed in copiers and printers having optical sensing devices that detect only opaque imaging materials, so opaque paper or film must be used to render part of the transparent imaging sheet opaque. Typically, a removable and repositionable opaque pressure-sensitive adhesive (PSA) paper or film stripe is placed on one edge of the non-imaging surface of the transparent imaging sheet. The PSA paper stripe is usually discarded after imaging so that the imaged sheet can be completely viewed.
Various conventional adhesives are used to coat the PSA paper stripe so that it can adhere to the transparent imaging sheets, but two problems have been found with conventional adhesives.
One problem is that the adhesion between the PSA paper stripe and the transparent imaging sheet increases over time causing the stripe to tear on removal. The base transparent films used to make transparent imaging sheets are typically polyester films such as poly(ethylene terephthalate) (PET) films. The imaging side of the base PET film being is coated with an ink or toner receptive coating. The non-imaging side of the base PET film is either not coated, or coated with a back-coating (primer) for better sheet handling. (In some instances, the transparent imaging sheets are coated with an ink or toner receptive coating on both sides.) The adhesive on the paper stripe may interact with the coating on the non-imaging side of the PET films at elevated temperatures (such as temperatures occurring in a copier machine), resulting in increased peel strength between the stripe and PET film. When a person attempts to remove the paper stripe from the PET film, the stripe may tear due to this increased peel strength.
In order that the PSA paper stripe can more easily be removed, microsphere adhesives are used in making the adhesive for the stripe. In microsphere adhesives, inorganic and organic microspheres are incorporated into an adhesive system. Microsphere adhesives are known in the art.
For example, Merrill, Jr. et al., U.S. Pat. No. 3,857,731 discloses a pressure-sensitive sheet material comprising a substrate having a binder matrix bonded to at least one of its surfaces. Tacky elastomeric copolymer microspheres protrude from the exposed surface of the binder. The microspheres are prepared by an aqueous polymerization technique utilizing emulsifiers and contain 95 to 99 weight percent acrylate monomer and 5 to 1 weight percent ionic monomer, maleic anhydride, or a mixture thereof. The binder compound is compatible with the microspheres. Relatively hard resins such as epoxies and nitrocellulose and relatively soft resins such as acrylates and vinyl ethers are described as being suitable binder compounds.
Loder et al., U.S. Pat. No. 4,049,483 discloses a hot melt adhesive system which has pressure-sensitive characteristics at room temperature comprising a heat-activatable hot melt adhesive containing therein inherently tacky elastomeric microspheres.
The second problem with conventional adhesives is their low cohesive strength. Paper stripes coated with the adhesives described above, can meet the peel strength requirement for transparent imaging sheets (e.g. easy removal from a transparent imaging sheet). However, stripes with this type of adhesive will leave residue on the imaging sheet after imaging or over long periods of time with no imaging. The residue appears as a dark stripe on the imaging sheet in overhead projector presentations and is aesthetically undesirable. The problem is likely due to the low cohesive strength of the adhesive, allowing microspheres in the adhesive to migrate to the imaging sheets and form residue. The following patents describe the possible use of cross-linking agents to improve the cohesive strength of certain adhesive systems.
Goetz, U.S. Pat. No. 5,626,955 discloses a pressure-sensitive adhesive tape, that employs a water-absorptive backing, a microparticle-containing pressure-sensitive adhesive, and a water-dispersible component. The microparticle component is comprised of polymeric, elastomeric, solvent insoluble but solvent dispersible microparticles. The patent discloses that these spheres may be inherently pressure-sensitive and cross-linked if desired.
Cooprider et al., U.S. Pat. No. 5,571,617 discloses a coated sheet material comprising a backing and a coating of a repositionable pressure-sensitive comprising a plurality of solid microspheres, a polymeric stabilizer, and a surfactant. The microsphere adhesive composition may also contain a crosslinking agent. The patent describes useful crosslinking agents as multifunctional (meth)acrylates and divinylbenzene.
Delgado et al., U.S. Pat. No. 5,266,402 discloses a pressure-sensitive adhesive comprising an acrylate matrix and swellable elastomeric acrylate microspheres. Useful microspheres are described as being hollow swellable elastomeric microspheres having discrete boundaries and made by conventional processes. The pressure-sensitive adhesive is made by mixing the elastomeric microspheres into an alkyl acrylate monomer and then adding a copolymerizable monomer and initiator, forming an interpenetrating polymer network within the boundaries of the microspheres. The patent discloses that when superior cohesive strengths are desired, the adhesive matrix may also be cross-linked and that multiacrylates such as 1,6 hexanediol diacrylate are preferred cross-linking agents.
Darvell et al., U.S. Pat. No. 4,855,170 discloses pressure-sensitive adhesive tape products. The tape products are made by applying to a sheet backing, a tacky adhesive which contains and completely surrounds hollow, resilient polymeric microspheres. The patent discloses that the specific type of pressure-sensitive adhesive used is not critical and can be of the rubber-resin type or the so-called "acrylate" adhesives. The patent discloses that the polymeric microspheres can be added to the adhesive matrix after the adhesive matrix has been polymerized, or prior to polymerization of the adhesive matrix. The patent also discloses that conventional adhesives such as dyes, pigments, fumed silica, chopped fibers, hollow glass microspheres, fillers, catalysts, cross-linking agents, and the like can be added to the adhesive.
In view of the foregoing problems with conventional adhesives on transparent imaging films, there is a need for a pressure-sensitive adhesive article having high cohesive strength which can be easily removed from imaging sheets. The present invention provides such adhesive articles.